realloc & Growing Accounts

Programs can grow (or shrink) account data via the realloc syscall/CPI, paying additional rent from a payer. Required when schemas grow or dynamic collections expand.

Recipe

account.realloc(new_len, false)?;
account.transfer_required_rent(payer)?; // conceptually - use realloc CPI pattern

When to reach for this:

• Adding fields in program upgrade.
• Growing dynamic vector storage.
• Shrinking on data prune.

Working Example

// Native: use solana_program::program::realloc or system CPI patterns per SDK
// After realloc, zero new bytes and initialize new fields
let new_len = old_len + 8;
// realloc account to new_len with payer funding rent delta

What this demonstrates:

• Rent delta must be funded.
• New bytes must be initialized.
• Shrink returns rent to destination.

Deep Dive

Safety

• Reject shrink if would truncate live fields.
• Zero new region to avoid leak of stale bytes.

Anchor

realloc constraint in 0.32.1.

Rust Notes

// Document max_len to cap griefing.

Gotchas

• Realloc without payer funds - Fails mid-upgrade. Fix: Ensure payer signer with lamports.
• Uninitialized new bytes - Deserialize garbage. Fix: Zero init.
• Shrink active data - Corruption. Fix: Forbid or migrate first.
• Unbounded growth - Rent griefing. Fix: Cap max_len.
• Realloc in loop - CU expensive. Fix: Batch growth.

Alternatives

FAQs

Related

• Data Versioning & Migration - Schema
• Space Calculation - Planning
• Upgrade-Safe Data Design - Forward compat

Stack versions: This page was written for Agave 4.1.1, Solana CLI 3.0.10, Anchor 0.32.1, anchor-lang 0.32.1, Rust 1.91.1, @solana/kit 7.0.0, Surfpool 0.12.0, and LiteSVM 0.6.x.